This invention relates generally to a control device for a hydraulically operated load and more particularly, it relates to a load independent control device for regulating flow of a hydraulic fluid including at least one multiway piston valve which has a housing and, within the housing, a longitudinal boring for slidably guiding the valve piston between a neutral position and a plurality of control positions, a fluid inlet channel connectable to a pump, a load channel connectable to a load, a fluid return channel connectable to a tank, the piston including at least one choke channel for selectively connecting in one position thereof the control inlet channel to the load channel and in another position thereof the load channel to the return channel, a spring-biased piston manometer valve connected between the inlet channel and the return channel, and being additionally biased by the pressure difference on a choke to maintain a constant pressure difference in individual channels, a control conduit connected between the inlet channel and the return channel and including a branch conduit to additionally bias the piston of the manometer control valve.
From prior art a control device of this type is known in which the piston of the multiway valve can take a neutral position and only two working positions in which a first control conduit is fully activated whereas choking points are operated in response to the movement of the piston to choke the flow proportionally to the displacement of the latter. At the same time, the load pressure is always transmitted via another control conduit to a piston manometer valve. The disadvantage of this known control device is the fact that the other control conduit to the load cannot be fully shut off and consequently the choking action in the first control conduit in the range of a fine adjustment of the device is impossible. It is also impossible to increase the effective pressure difference to attain maximum working fluid flow to the load. Another disadvantage of this known device results from the streaming of the working oil to the load via choking non-return valves. This choked streaming in the case of a parallel operation of several loads causes a limited unloading and leads indeed to an additional energy consumption (British Pat. No. 1,401,602).
Furthermore, a control device for a hydraulic load having a load independent flow regulation is known which is designed without any control conduit from the pump channel communicating via a choke and a choke channel in the piston with the tank. The omission of this control conduit means that in the end positions of the piston the influence of the load compensation cannot be fully eliminated and consequently the piston manometer valve is not exposed to the full pump pressure in lieu of the load pressure but to an intermediate pressure which more or less deviates from the pump pressure. This intermediate control pressure is to be applied to the manometer piston via a branched narrow channel system. It is true that this arrangement permits in the end position of the piston of the multiway valve to apply full pump stream to the load provided that the spring biasing the piston of the manometer valve is correspondingly strong; nonetheless, this design results in considerable losses of energy in the neutral position of the piston. Moreover, the construction of this prior art control device necessitates a hollow piston with non-return valves arranged in its interior and thus the whole structure is relatively expensive. In addition, this control device has the disadvantage that the load pressure due to the location of the non-return valves is tapped off at the point where the pressure is reduced about the pressure difference caused by the non-return valves and consequently the piston valve manometer has to be adjusted for correspondingly higher pressures with the result that additional energy losses take place (German Pat. No. 1,959,764).